Time Capsule + USB transfer speed

I have a 500GB HDD attached to USB port of my 2TB time capsule. Once I drag some files from this 500GB HDD to the time capsule, will it transfer directly through the USB connection or will it transfer files over the wireless network to my mac (using in a cache fashion) and send it to the time capsule?

I'm asking this because I found that transfers from external USB to time capsule drive are extremely slow